1. Field of the Invention
The invention relates to wireless communication, specifically to retrieving frequency information from a received wireless transmission.
2. Description of the Related Art
Wireless communication has become one of the most common ways of transmitting information from one place to another. The application of wireless communication links to telephone systems, local area computer networks, and wide area computer networks such as the Internet is widespread, and a large number of commercial manufacturers supply wireless communication equipment to these markets.
Some fundamental principles of a conventional wireless communication technique are illustrated in FIG. 1. In some forms of wireless communication, a baseband signal 10, which encodes a selected message, is combined with a high frequency sine wave carrier signal in an upconverter 14. In FM transmission, for example, the frequency of the carrier signal is altered in a way defined by the content of the baseband message signal 10. The modulated carrier 16 is then transmitted, and is received by a downconverter circuit 18 at the appropriate message destination. Both the upconverter 14 and the downconvertor 18 have reference frequency signals 12, 20 as inputs to perform the upconversion and downconversion processes. In some cases, the baseband message signal 22 produced by the downconverter 18 is a reproduction of the original message signal 10, but this need not be the case, as long as the message content is decodable from the downconverter 18 output.
In a broadcast system, such as broadcast television, only a central station will include circuitry for signal transmission. However, for two-way communication between, for example, a central hub and a number of remote units, the hub and the remote units will all include both downconversion circuitry for receive operations, and upconversion circuitry for transmit operations. In this case the remote units must also generate an upconversion reference frequency for message transmission to the hub, although for two-way communication, the carrier frequencies may differ depending on message direction.
In a large number of communication systems such as the one shown in FIG. 1, it is important that the frequency of the upconversion reference signal 12 and the frequency of the downconversion reference signal 20 be consistently and accurately related to one another to produce an easily interpreted downconverter output 22 and to allow optimal use of the allocated frequency. In addition, government regulatory agencies require the frequency of the carrier signal used for transmission to be equal to a specific assigned frequency within a very tight tolerance.
These goals are sometimes achieved by installing highly accurate and stable oscillators at both ends of the link to generate the reference signals 12, 20 at the link ends. One example of such a system for two-way communication is illustrated in FIG. 2. In this system, a signal received by a satellite dish 23 is downconverted by a series of two mixers 24a, 24b. The first mixer 24a may reside in an outdoor mounted unit 26, and downconverts the center frequency of the received signal to an intermediate frequency (IF). The second mixer 24b may reside in an indoor mounted unit 28, and downconverts the IF signal to a modulated baseband output which is sampled by an analog to digital converter 30. The output of the analog to digital converter 30 is routed to digital processing circuitry to demodulate the baseband signal and reproduce the bits which comprise the received data.
During the transmit process, the output of a signal generator 32 is upconverted by another pair of mixers 33a, 34b, the first one of which may be housed in the indoor unit 28, and the second one of which may be housed in another outdoor unit 34.
The mixers each receive a reference frequency signal from an associated phase-locked-loop (PLL) 36a, 36b, 36c, 36d. Each PLL in turn receives a reference frequency from an oscillator 37 from which it derives its output frequency. A controller 38 is coupled to the PLLs for digitally setting internal loop dividers so that each PLL can output a different frequency signal even though they all receive a common reference frequency from the oscillator 37.
In this type of system, the oscillator 37 is free-running, and should be as accurate and stable as possible over time, with temperature variations, etc. Typically, free running oscillators of sufficient quality to serve this function are among the most expensive components of the system. In addition, even with a high quality oscillator, some offset from an ideal reference frequency is present. In some systems, these offsets may be detected by analyzing the characteristics of the baseband receive signal during bit decoding. Deviation from 0 Hz center frequency in the baseband signal is measured, and is attributed to an offset from an ideal output of the oscillator 37. Once this offset is quantified during receive operations, the loop dividers in the transmit PLLs 36b, 36c can be updated to make these PLL outputs accurate even in the presence of the detected oscillator 37 offsets.
In some systems, one end of a two-way communication link can generate appropriate reference frequencies directly from a modulated carrier signal received from the other end of the link. However, the circuitry required to extract a stable reference frequency from a received modulated carrier is also expensive. Thus, less expensive techniques for generating these reference frequencies are needed as wireless communication expands into more widespread use.
The invention comprises methods and apparatus for frequency generation in communication systems. In one embodiment, a communication device according to the invention comprises upconversion circuitry, downconversion circuitry, and a reference frequency generator. In this embodiment, the reference frequency generator is coupled between an output of the downconversion circuitry and the upconversion circuitry. The reference frequency generator may comprise a voltage controlled oscillator and a phase comparator.
Methods of generating an upconversion frequency signal for a transmitter are also provided. In a wireless communication apparatus comprising a transmitter and a receiver, such a method may comprise extracting a modulation frequency from a signal received by the receiver, and deriving the transmit upconversion frequency from the modulation frequency.